Replace usages of MobObserver and AdminObserver with GameTicker consts (#21814)

Content.Server/GameTicking/GameTicker.GamePreset.cs

@@ -262,7 +262,7 @@ namespace Content.Server.GameTicking
             var xformQuery = GetEntityQuery<TransformComponent>();
             var coords = _transform.GetMoverCoordinates(position, xformQuery);
 
-            var ghost = Spawn("MobObserver", coords);
+            var ghost = Spawn(ObserverPrototypeName, coords);
 
             // Try setting the ghost entity name to either the character name or the player name.
             // If all else fails, it'll default to the default entity prototype name, "observer".